1. Field of the Invention
The present invention generally relates to a technology for forming images, and particularly relates to forming images using a two-component developer.
2. Description of the Related Art
A two-component developing method has been known in which a two-component developer (hereinafter “developer”) that contains a non-magnetic toner and a magnetic carrier is held on a developer holding member to form a magnetic brush by a magnetic pole inside the developer holding member, and a latent image formed on a latent image holding member is developed by the magnetic brush into an image. The two-component developing method is in widespread use because of the easy colorization. According to the two-component developing method, when toner concentration, i.e., the ratio (for example, weight ratio) of toner to magnetic carrier contained in the developer, is too high, an image may be smudged in the background or resolution may be lowered in detailed parts of the image. On the other hand, when the toner concentration is too low, the density of solid areas in the image may be lowered or carriers may adhere to the latent image holding member. Therefore, the toner concentration in a developer needs to be controlled and ensured to be always within an appropriate range in such a manner that the toner concentration is detected and the toner supply operation is controlled in a developing device.
Generally, the toner concentration is detected by the amount of toner or the number of magnetic carriers in a two-component developer present in a predetermined detection area in the developing device. A typical example of this method uses a magnetic permeability sensor (a detecting unit). The magnetic permeability sensor recognizes magnetic characteristics of magnetic carriers contained in a developer present in the predetermined detection area as an electric signal (frequency, voltage, etc), and outputs the electric signal. When the toner concentration is within a practical range, the output value of the magnetic permeability sensor monotonically decreases as the number of the magnetic carriers present in the detection area increases. Based on the output value, the toner concentration in the developer can be detected.
However, with the method described above, when there is a change in the bulk density of the developer in the detection or the fluidity of the developer, the output value of the magnetic permeability sensor also changes even if the toner concentration is unchanged. In such a situation, the toner concentration indicated by the output value of the magnetic permeability sensor is different from the actual toner concentration.
Japanese Patent Application Laid-open No. 2003-280355 discloses a conventional image forming apparatus that uses a magnetic permeability sensor to detect toner concentration in a developer in a developing device and compares the output value of the magnetic permeability sensor with a target output value, thereby controlling the toner concentration. The conventional image forming apparatus has image forming modes in each of which image forming is performed at a different process linear velocity. When the image forming mode is switched from one to another, the process linear velocity is changed, and the developer stirring/carrying speed in the developing device is also changed. Consequently, the number of magnetic carriers in the detection area of the magnetic permeability sensor per unit of time varies depending on the image forming mode. As a result, even if the toner concentration is unchanged, the output value of the magnetic permeability sensor varies depending on the image forming mode.
In the conventional image forming apparatus, the process linear velocity is set at a standard linear velocity in a warm-up period, and the toner concentration is controlled to an appropriate level at the standard linear velocity. In other words, the output value of the magnetic permeability sensor is controlled to a target output value. Subsequently, control voltages to be applied to the magnetic permeability sensor are set so that the output values for toner concentration levels each corresponding to one of the three image forming modes is the target output value, the three image forming modes being preset to have mutually different process linear velocities. When image forming is performed in one of the image forming modes, a control voltage corresponding to the image forming mode is applied to the magnetic permeability sensor, and the toner concentration is detected to control the toner concentration in a developer. With the conventional image forming apparatus performing such control, no matter in what image forming mode image forming is performed, it is possible to achieve the same output value of the magnetic permeability sensor as long as the toner concentration is the same.
According to the conventional technology described above, however, a developing device in which a two-component developer is used, and especially in a color image forming apparatus, an additive such as silica or titanium oxide is externally added to the surface of toner to improve the dispersion of the toner. Such an additive is easily affected by mechanical stress or thermal stress. During the stirring process in the developing device, the additive may be embedded in the toner or released from the toner surface. As a result, the fluidity or the charging characteristic of the developer changes, and the bulk density of the developer also changes.
In addition, in the course of time, due to a change in the shape of the magnetic carrier surface, accumulated external additives removed from toner, or a decrease in the chargeability of magnetic carrier (called “CA”) due to peeling of a carrier coating film, the fluidity of the developer changes, and the bulk density of the developer also changes.
These changes prevent the magnetic permeability sensor from detecting the toner concentration accurately. For example, when an image forming apparatus has a plurality of image forming modes, and the developer stirring/carrying speed in the developing device varies depending on the image forming mode, the output value of the magnetic permeability sensor changes even if the toner concentration is unchanged as explained above. Further, the correction amount for the output value of the magnetic permeability sensor changes according to degradation or use status of a developer. Consequently, there has been a difficulty in accurately correcting the output value of the magnetic permeability sensor.